Does natural selection ever paint a species into a corner, leaving it with anatomy so specialized that a slight change in its environment pushes it over the edge into extinction? Consider the 30-foot-long (nine meters)* diplodocoid (a branch of the sauropod group) Nigersaurus—an anatomical side-show with a mouth shaped like a vacuum cleaner, hundreds of tiny teeth, a boom of a neck, and skull bones thin to the point of translucence. How did it survive with such a preposterous eating apparatus? Paleontologist Paul Sereno, a National Geographic explorer-in-residence, picked up the quizzical beast's trail in the mid-1990s in northeastern Niger. Nigersaurus's oddest feature is its broad, straight-edged muzzle, which allowed the business end of its mouth to work very close to the ground, "like a lawn mower," says Sereno. Its more than 500 teeth, each about the size of a toddler's incisor, were tightly aligned, with a single row of more than 50 in operation in each jaw at any one time. A CT scan exposed up to eight "replacements" stacked up behind each tooth, so that new teeth immediately filled in for worn ones. Despite its impressive battery of teeth, Nigersaurus had a weak bite. Where the jaw muscles attach to the skull, the bone is as thin as a paper plate.

"Its mouth appears designed for nipping rather than chomping or chewing," says Sereno, pointing to wear patterns that suggest the teeth slid by one another like a pair of shears. With nearly 80 percent of the animal's skeleton recovered, a portrait emerges of a finely tuned eating machine designed to crop mouthfuls of soft plants growing near the rivers that coursed through what is now the Sahara's southern flank. Its long neck would have allowed Nigersaurus to mow down an entire field of plants without taking a single step.

A number of these features, seen in the extreme in Nigersaurus, show up over millions of years in other diplodocoids, which thrived on nearly every major landmass. That, says Sereno, suggests that this feeding strategy emerged in primitive form much earlier. But could this evolutionary trend toward the perfect eating machine ultimately have led to the extinction of the lineage? "Selection can favor specialization that can be advantageous over the short run, but create vulnerabilities over the long haul," says University of Chicago evolutionary biologist David Jablonski. We'll never know for sure whether Nigersaurus fell victim to its own outlandishness. But while it lasted, this fern-eating giant was a bizarre and beautiful success.

* After the print edition of National Geographic magazine's December issue went to press, scientists reduced their estimates for Nigersaurus's length from some 50 feet (15 meters) nose to tail to 30 feet (nine meters) and the number of teeth from approximately 600 to around 500.